It depends of the finish you want.
If you don't prewash fabric, don't prewash the batting. It should shrink at almost the same rate. If you do prewash fabric, you have a choice to make.
I prewash fabric and not the batting. I like the look after it's washed. I've used Warm & Natural and Warm & White, and have a wool waiting on the sidelines to try - excited.